Fiber-reinforced materials in the form of prepregs are already used in many industrial applications because they are convenient to handle and because of increased processing efficiency in comparison with the alternative wet lamination technology (“wet-lay-up” technology).
Demands of industrial users of systems of this type are not only good handling but also longer shelf lives and lightfastness, and also shorter cycle times, and prepreg-hardening temperatures that are low and more energy-efficient.
This requires matrix components that permit the production of prepregs that can be stored and that have properties sufficiently stable for further processing. To this end, the prepregs cannot be tacky. It is moreover not permissible that they have been fully hardened. It is permissible only that the resin matrix has been prepolymerized, i.e. it must remain fusible. Requirements placed upon the crosslinked resin matrix consist in a high level of adhesion at interfaces in respect of the reinforcing materials and insert components, and where appropriate also in respect of other materials, such as metallic or ceramic materials. In the crosslinked state there are also requirements for high chemical stability and heat resistance.
Alongside polyesters, vinyl esters, and epoxy systems there are many specialized resins in the field of crosslinking matrix systems. Among these are also polyurethane resins, which are used by way of example for the production of composite materials by way of SRIM (structural reaction injection molding) processes or pultrusion processes because they are tough, damage-tolerant, and robust. Polyurethane composites also have superior toughness in comparison with vinyl esters, unsaturated polyester resins (UPE), or UPE-urethane hybrid resins.
Prepregs and composite components produced therefrom, based on epoxy systems, are described by way of example in WO 98/50211.
WO 2006/043019 describes a process for the production of prepregs based on epoxy resin polyurethane powders.
DE-A 102010029355 describes a process for the production of storage-stable polyurethane prepregs, and describes moldings produced therefrom, these being obtainable via a direct-melt-impregnation process from fiber-reinforced materials with use of reactive polyurethane compositions. The in essence aliphatic polyisocyanates used here are either internally blocked (e.g. as uretdione) and/or blocked by external blocking agents. The reactive resin mixtures can be used at temperatures of from 80 to 120° C. in the direct-melt-impregnation process. The disadvantage is that the hardening temperature is from 120° C. to 200° C., depending on the system, and the hardening time/cycle time is very long, being up to 60 minutes, with resultant high energy costs and high production costs. The examples use a leveling additive, and it can therefore be assumed that the systems described have high viscosities.
WO 2013/139704 A1 describes impregnation of reinforcing fibers with a very low-viscosity polyurethane system with high index, for the production of PUR prepregs that are storage-stable but nevertheless reactive. These prepregs have the disadvantage that they are unsuitable for weathering-resistant, lightfast applications. Another disadvantage is that the processing time is dependent on the mass of the mixture produced. An increase in the mass of the mixture leads to a reduction of processing time (potlife). Potlife is the period between mixing of the components and impregnation of the reinforcing fibers by the as yet not fully reacted matrix material.
There are also known prepregs based on pulverulent thermoplastics as matrix. US-A 20040231598 describes a method in which the particles are passed through a specific acceleration chamber with electrostatic charging. This apparatus serves for the coating of glass substrates, aramid substrates, or carbon-fiber substrates for the production of prepregs from thermoplastic resins. Resins mentioned are polyethylene (PE), polypropylene (PP), polyetheretherketone (PEEK), polyether sulfone (PES), polyphenyl sulfone (PPS), polyimide (PI), polyamide (PA), polycarbonate (PC), polyethylene terephthalate (PET), polyurethane (PU), polyester, and fluoropolymers. The thermoplastic prepreg textiles produced therefrom exhibit inherent toughness, good, viscoelastic damping behavior, unrestricted shelf life, good chemicals resistance, and recyclability.
Composite components with a matrix based on 2-component polyurethanes (2-C PUR) are likewise known. The 2-C PUR category comprises in essence the traditional reactive polyurethane resin systems. In principle, the system has two separate components. Whereas the main constituent of one of the components is always a polyisocyanate, the main constituents of the second component are polyols or amino- or amine-polyol mixtures. The two parts are mixed with one another only briefly prior to processing. The chemical hardening reaction then takes place via polyaddition with formation of a network made of polyurethane or polyurea. 2-C PUR systems have a restricted processing time (potlife) after the mixing of the two constituents, since the exothermic reaction that begins leads to gradual viscosity increase and finally to the gelling of the system. There are numerous variables here that determine the effective time available for processing: reactivity of the reactants, catalysis, concentration, solubility, moisture content, NCO/OH ratio, and ambient temperature being the most important [Lackharze [Coating resins], Stoye/Freitag, Hauser-Verlag 1996, pp. 210/212]. The disadvantage of the prepregs based on 2-C PUR systems of this type is that there is only a short time available for the processing of the prepreg to give a composite. Prepregs of this type are therefore not storage-stable over a plurality of hours, or indeed days.
JP-A 2004196851 describes composite components which are produced from carbon fibers and from organic fibers, e.g. hemp, with use of a matrix made of 2-C PUR based on polymeric methylenediphenyl diisocyanate (MDI) and on specific compounds containing OH groups.
WO 2003/101719 describes polyurethane-based composite components and methods for producing same. 2-C polyurethane resins are involved, with defined viscosities in the range from 300 to 2000 mPas, and with particular gel times of from 3 to 60 minutes.
There are also known physically-drying systems based on non-reactive PUR elastomers. Relatively high-molecular-weight, linear, thermoplastic polyurethanes are involved here, derived from diols and from diisocyanates, preferably MDI, TDI, HDI, and IPDI. These thermoplastic systems generally have very high viscosities, and therefore also have very high processing temperatures. This greatly increases the difficulty of use for prepregs. The use of powders in reactive systems in the production of prepregs with fiber composites is rather unusual, and has hitherto been restricted to a small number of application sectors.
Probably the most commonly used process for applying a powder to a fiber surface is the fluidized bed process (fluidized bed impregnation). Powder particles subjected to an upward-directed flow pattern assume fluid-like properties. This method is used in EP-A 590702. Here, individual fiber bundles are opened to release the strands, which are coated with the powder in the fluidized bed. The powder here is composed of a mixture of reactive and thermoplastic powder, in order to optimize the properties of the matrix. Individual rovings (fiber bundles) are finally brought together, and a plurality of layers are pressed for about 20 minutes at a pressure of 16 bar. The temperatures vary between 250 and 350° C. However, irregular coating is frequently encountered in the fluidized-bed process, in particular when the strands are not completely separated from one another.
In this connection, US-A 20040231598 presents a method which functions similarly to the fluidized bed process. Here, an air stream transports the particles to the substrate, and a specific structure is used for uniform deposition of the powder.
DE-A 102009001793 and DE-A 102009001806 describe a process for the production of storage-stable prepregs in essence composed of at least one fibrous support and of at least one reactive pulverulent polyurethane composition as matrix material.
WO 2012/022683 describes fiber-composite components and a process for production of these. The polyurethane used to saturate the fiber layer is produced from a reaction mixture. The reaction mixture comprises, as essential constituent, one or more polyepoxides, alongside polyisocyanates, polyols, and optionally additives. The polyurethane described in said document has the disadvantage of shelf life that is not adequate for the production of prepregs, being characterized by way of example by a low glass transition temperature. This PUR system moreover does not have the NCO value required for postcrosslinking to give finished components.